In the majority of hot gas engine embodiments, the heating and cooling of the working gas takes place outside the cylinders. Thus, the working gas contained in the volume swept by the piston does not get properly heated during the expansion and properly cooled during compression. Hence, the actual cycle in these embodiments is different from either the Stirling or the Ericsson cycles and these hot gas engines cannot achieve Carnot efficiency. There are hot gas engines where the heating and cooling regions are incorporated within the cylinder volumes swept by the pistons. However, in these embodiments quantities of the working gas continuously cross over from the hot cylinder to the cold cylinder while the expansion is in progress, and from the cold cylinder to the hot cylinder while the compression is in progress. It can be shown that the working gas that is present in the cold cylinder during each instant that the expansion is in progress, and the working gas that is present in the hot cylinder during each instant that the compression is in progress contribute negative work cycles that reduce the thermal efficiency of the engine from the Carnot efficiency.
It is therefore desirable to provide a hot gas engine whose thermal efficiency is maximized by ensuring that virtually all the working gas that was in the hot cylinder at the start of the expansion step remains in the hot cylinder for the duration of the expansio step, and all the working gas that was in the cold cylinder at the start of the compression step remains in the cold cylinder for the duration of the compression step. Also, isobaric working gas transfer from cylinder to cylinder is optimized by making the hot and cold cylinder volumes such that they are in the same ratio as the absolute temperatures of their isothermal processes.
Additionally, in the hot gas engines with heating and cooling surfaces provided within the cylinder volumes swept by the pistons, the heat transfer surface is generated by having piston projections which mesh with coresponding noncontacting depressions in the cylinder housing. In order to generate the quantities of heat transfer area that are generally required, the number of projections becomes inordinately large. This large number of projections results in void volume within the cylinders which adversely affects the thermal efficiency and engine performance.
In the proposed invention embodiment the number of projections is limited to control the void volume; however each projection is made up of laminations that are made to separate from each other when the piston descends from its top dead center position, to generate large additional heat transfer areas for transfer of heat into or from the expanding or compressing working gas respectively.